FalconSat-3 is the 3rd student-built microsatellite demonstration mission of the USAFA (United States Air Force Academy), Colorado Springs, CO, USA. The objective is to demonstrate innovative, highly-efficient boom deployment technology. Work on the FalconSat-3 project started in 2002. 1)2)3)4)5)6)

The use of booms on small satellites represent an important mission capability enhancement which may serve as carriers for various tip payloads. The EMC (Elastic Memory Composite) materials are a relatively new addition to the family of shape memory materials. Key advantages of EMC materials over conventional shape memory alloys and shape memory ceramics boom designs are their substantially lower densities, higher strain capacities, and higher damping capabilities.

The FalconSat-3 microsatellite mission is dedicated to validate next-generation boom deployment techniques for payload tip mounts and for use of passive gravity gradient stabilization of the spacecraft. The AFRL (Air Force Research Laboratory) is providing funding for the development of the so-called MPACS (Micro-Propulsion Attitude Control System) program on FalconSat-3. The total boom mass is 10.6 kg with a tip mass of 8 kg.

The design considers a 3.3 m long TEMBOTM EMC deployable boom structure of CTD (Composite Technology Development) Inc., Lafayette, CO. These laminates are to be folded in a serpentine fashion within the canister for launch, and heated for deployment, once on-orbit. 7)

FalconSat-3, also using the COTS (Commercial Off-the-Shelf) hardware approach like its predecessor FalconSat-2, is a spacecraft with a total mass of 54.3 kg, a gravity-gradient boom stabilized satellite that carries three main and two secondary DoD-funded experimental payloads - MPACS (Micro Propulsion Attitude Control System), FLAPS (Flat Plasma Spectrometer), and PLANE (Plasma Local Anomalous Noise Environment). 8)9)

The FalconSat-3 project selected a COTS spacecraft bus manufactured by SpaceQuest Ltd. of Fairfax, VA. The avionics stack, consisting of seven "modules" of mechanically integrated components, has external dimensions of approximately 23.6 cm x 23.6 cm x 19.85 cm. The seven modules that make up the avionics stack include the following module trays: BAT (Battery Module), BCR (BCR-100 Module), IFC (Integrated Flight Computer-100 Module), Tx (RF Transmitter Module), TQR (ADCS Torquer Integration Module), Rx tray (RF Receiver Module), and PIM (A/D Converter Module). The use of COTS hardware has provided the opportunity for the cadets and faculty to focus on the unsolved problems by providing an almost complete avionics solution.

Figure 4: View of the TQR tray (image credit: SpaceQuest)

Deployable boom concept: Introduction of shape memory "mechanisms" by using the innovative EMC (Elastic Memory Composite) material design. The TEMBOTM line of EMC materials are heat-activated (exploiting the transition in the mechanical properties of the material that occur at the critical laminate temperature). The laminate properties below this critical temperature are comparable to the conventional "rigid" composite structures. This concept allows for strain energy to be "frozen" into an EMC structure by cooling the deformed laminate below the critical temperature. 10)

The TEMBOTM EMC family of booms has been designed around a common central element, EMC longerons with embedded heaters. These longerons are both the primary deployment mechanism and the principal structural members of the boom. The tip mass of the boom features MPACS (Micro Propulsion Attitude Control System) which is being used for attitude control of the spacecraft.

Figure 5: Block diagram of FalconSat-3 (image credit: USAFA)

Launch: A successful launch of FalconSat-3 took place on March 9, 2007 (UT) on an Atlas-5-401 vehicle from the Cape Canaveral Air Force Station. The spacecraft was released at T+66 minutes and 5 seconds. FalconSat-3 is a secondary payload on the STP-1 (Space Test Program-1) mission of DoD with the EELV (Evolved Expendable Launch Vehicle) of Atlas-5, using the ESPA (EELV Secondary Payload Adapter). The primary payload on this flight is OE (Orbital Express). The other secondary payloads are: STPSat-1, CFESat, and MidSTAR‐1.

Orbit: Circular orbit, altitude = 560 km, inclination = 35.4º.

Note: The STP‐1 mission had to deal with the deployment of 5 satellites into two orbital planes at two different altitudes. The Orbital Express (prime payload) and MidSTAR‐1 spacecraft were deployed in the first orbital plane at an altitude of 492 km and an inclination of 46º. After two more centaur burns, the remaining ESPA payloads, STPSat‐1, NPSat‐1, CFESat, and FalconSat‐3, were inserted into the second orbital plane at an altitude of 560 km and an inclination of 35.4º.

• September 25, 2017: AMSAT has reported that the U.S. Air Force Academy satellite FalconSAT-3 is now open for amateur radio use as a digital store-and-forward system. 11)

- The satellite is in a 35.4 degree inclination orbit, with an approximate altitude of 465 to 476 km. The Packet Bulletin Board System is operating at 9600 baud with a 145.840 uplink and 435.103 downlink. Output power is 1 W and the downlink is continuously on. Digipeating is enabled for live QSOs, but unattended digipeating operations are not authorized at this time. Current Keplerian elements can be found in the AMSAT distributed Keplerian elements. 12)

• In the summer of 2011, FalconSAT-3 has surpassed four years on orbit and is completing additional spaceflight experimentation tasks for AFRL. 13)

• On January 21, 2011, in a unique union, the US Air Force Academy's FalconSAT-3 satellite went in joint service with the United States Military Academy at West Point, New York. 14)

• FalconSat-3 is "operational" as of 2010 (however, the ADCS problem remains). 15)

It turns out that the orbits of FalconSat-3 and the Cibola spacecraft of LANL are very similar and pass close to each other every 225 to 250 days, remaining in close proximity for about two days during each pass. When this happened in mid-December 2009, Cibola's sensors were trained on FalconSAT-3 to help the FalconSAT team investigate a spacecraft altitude determination and control problem. 16)

In 2009, FlaPS remains in an active experimentation mode; however, due to an anomaly with the FalconSat-3 TT&C subsystem, only minor amounts of data have been made available for analysis. 17)18)19)20)

• As of fall 2008, FalconSat-3 has "operated" on orbit for almost fifteen months and is expected to last at least that much longer on orbit. The fundamental issues uncovered have in most cases been fixed and every one of these experiences has contributed significantly to the education of the students in the program.

Stabilizing the rate of rotation of FS-3 is critical in obtaining proper scientific data. Both FLAPS and PLANE payloads require facing the ram direction for accurate readings of the environment. Simultaneous FLAPS/PLANE operations will be conducted for a period of 6 months, followed by 1 month of simultaneous MPACS/PLANE testing. These experiments will then be followed up with more FLAPS/PLANE testing.

• A year and a half after launch, operators have nearly completed the task of stabilizing the satellite. The gravity gradient boom was deployed on November 29, 2007 and has mostly stabilized the satellite about the z-axis. Many software problems have arisen since launch. Problems have included inversed power settings on magnetorquers, non-working FLAPS payload, non-working sun-sensors, and improper time-stamping of telemetry data. The original control programs designed to stabilize the satellite, when implemented, tended to spin the satellite up further instead of dampening the rate of spin. After much data analysis, it was determined that the magnetorquers and magnetometers were interfering with each others successful operation and producing contamination within the control program. A multiplier was added to the control software to allow more time in between readings and firings.

Test are being run to further analyze a FLAPS software fix and determine whether starting a control program is beneficial. Although not completely stabilized, FS-3 has taught operators invaluable skills dealing with problem solving, spacecraft operations, and other space oriented tasks. The satellite is currently (fall 2008) rotating about the y-axis at 0.3 rpm. To obtain successful experimental data with FLAPS and PLANE, the rotation rate of FS-3 needs to be less than 0.1 rpm. Once the rate of spin is reduced, operators will conduct normal operations.

Resolution of the final ADCS and FLAPS interface problems will be solved by very carefully backing down the software requirements on the IFC and modifying ADCS software to accommodate the corrupted magnetometer data. The fundamental issue is one of far too many tasks required of the single IFC. This is a 286-class microprocessor without a hardware floating point capability and floating point calculations are done in an emulator. The original requirements for the satellite continued to grow after hardware selection was made and the goal now is to restructure some of the original code to work as well as possible within the limited cycles available.

• On Nov. 28, 2007, the gravity gradient boom was deployed.

• After launch it was not possible to contact the satellite for several weeks. The anomaly resolution process had started formally after failure to communicate with FalconSat-3 but no data of significance could be accomplished until there was some form of rudimentary communication. Finally, contact could be established on April 4, 2007. - With appropriate guidance from faculty and various professionals in the industry the cadets were able to thoroughly explore a variety of causes and slowly work through the problems and recover satellite operations.

Sensor complement: (MPACS, FlaPS, PLANE)

MPACS (Micro Propulsion Attitude Control System):

MPACS is a three-axis µPPT (Pulsed Plasma Thruster) system of Busek Inc., Natick, MA, funded by AFRL (Air Force Research Laboratory). The objective is to demonstrate attitude control. Four MPACS were integrated aboard FalconSat-3. 21)22)

The arrangement of the MPACS cluster (4 boxes) can be seen in Figure 1.

FlaPS is a collaborative instrument package of JHU/APL, NASA, and USAFA. The objective of FlaPS is to provide a solution to the investigation of plasma distributions in space - to characterize the effects of charged particles on the formation, propagation and decay of ionospheric plasma bubbles (a phenomenon that can disrupt satellite communications).

The instrument design (spectrometer) is based on MEMS (MicroElectroMechanical System) technology. FlaPS is capable of measuring the kinetic energy and angular distributions of ions/electrons in the LEO space environment for energies ranging from a few eV to 50 keV. Hence, the instrument is capable of providing the full neutral wind vector, full ion-drift velocity vector, neutral and ion temperatures, and deviations from thermalization. In addition, coarse mass spectroscopy is possible using an energy analysis technique. 23)24)25)26)27)28)

The suite of instruments is comprised of a set of 16 individual neutral and ion analyzers, each of which is designed to perform a specific function. Advances in miniaturization technology have enabled a design in which the 16-sensor suite resides on a circular microchannel plate with an effective area of 25 cm2. The FlaPS electronics package, consisting of low voltage and high voltage power supplies, a microprocessor, and ASIC (Application Specific Integrated Circuit) amplifiers, requires a volume of 290 cm3, power of 1.5 W, and a mass of 0.5 kg.

A collimator is used to select a look direction (represented by the colored cones in Figure 9) unique to each individual analyzer. The energy selector is then used as a band-pass filter to image plasma spectra differential in energy. The MCP (MicroChannel Plate) detectors provide charge multiplication to increase the sensitivity of the instrument. The anode and PADs (Pre-Amplifier Discriminators) are then used to collect the resulting current and process the signal that will be stored by the instrument electronics assembly.

The spectrometer's small size, low weight and power consumption, and increased resolution make it ideal and affordable for use in large numbers, and could be applied to other types of missions. The multi-organizational team is already working on the next-generation device known as WISPERS (Wafer-scale Integrated SPectrometERS), an instrument suite created by the same micro-electronics-based manufacturing techniques. WISPERS is is scheduled to fly on FalconSat-5 scheduled for launch in fall 2009.

PLANE (Plasma Local Anomalous Noise Environment):

PLANE was developed by USAFA to investigate the localized plasma environment around a spacecraft (to identify spacecraft-induced plasma turbulence). The PLANE experiment is a bifurcated retarding potential analyzer capable of distinguishing between ambient and spacecraft-induced turbulence. 29)

The PLANE experiment is a unique configuration of two planar RPAs (Retarding Potential Analyzers) linked by a feedback loop (the mechanical configuration is based on the RPA instrument developed for the MESA experiment on FalconSat-2).

Principle of operation:

• PLANE uses two retarding potential analyzers (RPA)

• Separate the signal from the turbulent lower energy from the higher ram energy ions

• Output from both instruments are differenced and monitored at high frequency

• Monitors turbulence to 10 cm scale size, a factor of 100 improvement over current techniques.

Figure 10: Prototype of PLANE (image credit: USAFA)

In addition to the three payloads, FalconSat-3 will carry a shock ring vibration suppression device developed by AFRL/VS and a shape-memory composite actuated gravity gradient boom also developed by AFRL/VS.

20) Martin E. B. France, William W. Saylor, "Undergraduate Small Satellite Programs at the U.S. Air Force Academy: Current Status and Future Directions," Proceedings of the International Workshop on Small Satellites, 'New Missions, and New Technologies,' SSW2008, Istanbul, Turkey, June 5-7, 2008

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.